1. Field of the Invention
This invention relates to a wavelength selection device, a wavelength selection laser, and a tunable laser, and more particularly to a wavelength selection device, a wavelength selection laser, and a tunable laser, each of which includes a wavelength selection filter for passing light of desired wavelengths and reflecting light of wavelengths other than the desired wavelengths, and makes use of light which passes through the wavelength selection filter and is then reflected by a reflection mirror arranged at the rear of the wavelength selection filter to return to the wavelength selection filter again.
2. Description of the Related Art
Recently, with a drastic increase in communication demands, development has been actively carried out on a wavelength division multiplexing system (WDM system) which enables large capacity transmission through a single optical fiber, by multiplexing a plurality of signal lights of different wavelengths. In such a WDM system, there is a great demand for a wavelength selection filter and a wavelength selection laser capable of selecting desired wavelengths from a wide range of wavelengths, to implement the system.
FIG. 25 is a conceptual representation of the wavelength selection laser which employs the wavelength selection filter. In one method of realizing the wavelength selection laser, a gain medium 202 covering a wide bandwidth and a wavelength selection filter 203 are arranged within a resonator formed by two reflection mirrors 200 and 201 arranged opposed to each other. In this method, when a filter, such as a Fabry-Perot (FP) etalon, which passes light of desired wavelengths and reflects light of wavelengths other than the desired wavelengths, is used as the wavelength selection filter 203, the light (selected light) that passes through the filter is resonated between the reflection mirrors 200 and 201, eventually causing laser oscillation to occur.
As the wavelength selection filter 203, it is possible to use a filter in which an FP etalon is formed by a quartz plate designed to have a predetermined thickness according to the wavelengths of light to be allowed to pass and having opposite surfaces thereof covered with predetermined reflection coating. There has been also proposed another conventional wavelength selection filter in which transparent substrates on a light-introducing side and a light-emitting side thereof are formed, e.g., by two sheets of polarizers, and two sheets of polarization rotators joined to outer sides of the two sheets of polarizers, for rotating respective polarization planes in opposite directions, and a liquid crystal is held with the transparent substrates via a spacer (see Japanese Laid-Open Patent Publication (Kokai) No. 11-125801). In this case, the polarization rotators, which are trapezoidal in cross section, are provided for varying polarized states of light transmitted through the polarization rotators, to thereby depolarize the incoming light, and the two sheets of polarization rotators are arranged such that respective directions in which the polarization planes are rotated thereby are made opposite to each other, whereby a wavelength selection filter is made capable of coping with incoming lights from two directions. Further, there is another example of the conventional wavelength selection filter which is configured such that two sheets of tabular mediums have reflection planes formed on respective one end surfaces thereof in a manner opposed to each other, and a spacer having a predetermined length is held between the reflection planes to form an FP etalon (see Japanese Laid-Open Patent Publication (Kokai) No. 2000-133863).
Now, the wavelength selection laser employing the wavelength selection filter suffers from a problem that when light (non-selected light) reflected by the wavelength selection filter without passing therethrough goes back into the resonator, it is impossible to produce stable laser oscillation at a desired wavelength. To overcome this problem, the wavelength selection laser is required to suppress the adverse effects of the non-selected light.
FIG. 26 is a diagram which is useful in explaining a conventional method of suppressing adverse effects of non-selected light. It should be noted that in FIG. 26, component parts and elements similar or equivalent to those of the wavelength selection laser shown in FIG. 25 are designated by identical reference numerals.
In the conventional method, to inhibit non-selected light reflected by the wavelength selection filter 203 from going back into the resonator, the wavelength selection filter 203 is arranged at an angle or in a manner inclined with respect to the optical axis. As a result of this configuration, selected light or component of light incident on the wavelength selection filter 203 passes therethrough, and is then reflected by the reflection mirror 201 to return to the filter 203 to pass therethrough. On the other hand, non-selected light is reflected by the wavelength selection filter 203, though in a manner deviated from the optical axis of the incident light or the selected light, because the wavelength selection filter 203 is arranged at an angle. Thus, the non-selected light is prevented from returning to the resonator.
However, the arrangement of the wavelength selection filter 203 at an angle with respect to the optical axis causes degradation of finesse and an increase in insertion loss, resulting in the degraded characteristics of the wavelength selection laser. Particularly, when the FP etalon of high finesse is inclined, e.g., at two degrees or more with respect to the incident direction, the finesse is degraded to a level equal to or less than 50% thereof, while the insertion loss thereof is increased up to 5 dB (decibels). This means that a wavelength selection laser designed to cause laser oscillation to occur at a wavelength selected by the wavelength selection filter, and further a tunable laser configured to cause laser oscillation to occur by selecting light of an arbitrary one of wavelengths selected by the filter are degraded in wavelength selectivity, which makes it difficult to cause stable laser oscillation.
Further, the arrangement of the wavelength selection filter at an angle with respect to the optical axis can cause an increase in size of the wavelength selection filter itself, the wavelength selection laser, or the tunable laser, employing the filter. Further, because fine adjustment of the angle of the wavelength selection filter is required, there remains another problem of troublesome alignment of the filter.